1. Field of the Invention
The present invention relates to current limiters or generators, and more specifically to current limiters likely to be inserted in power circuits to which high voltages, for example, above one hundred volts, can be applied.
2. Discussion of the Related Art
A current limiter is a component arranged in series in a circuit and intended for limiting the current in the circuit to a constant value. Thus, a current limiter is also often called a current generator.
Among current limiters or generators, one of the best known is formed of a depletion MOS transistor having its gate connected to its source. Such a component has the following disadvantages:                it incorporates an intrinsic diode in anti-parallel and thus cannot withstand reverse voltages;        it is technologically difficult to adjust the parameters which set the limiting current, especially the threshold voltage of the MOS transistor and the resistance of the preformed channel;        the silicon surface required to make the component is relatively large.        
Another simple circuit enabling realization of a current generator is shown in FIG. 1. This circuit is connected between terminals A and B, terminal A being normally positive with respect to terminal B, and includes the series connection of a transistor T and of a resistor R2. A resistor R1 is connected between the base and the collector of transistor T. A reference diode or avalanche diode Z is connected between the base of transistor T and terminal B. Emitter current Ie, which flows in resistor R2, is equal to (Vz−Vbe)/R2. If the values Vz, the voltage drop across the avalanche diode Z, and Vbe, the voltage drop between the base and emitter of transistor T, are substantially constant, then the current Ie is substantially constant.
This circuit however has the following disadvantages:
1) it is limited to the presence of low voltages, that is, the voltage between terminals A and B must not exceed a threshold value linked to the direct breakdown voltage of transistor T; this circuit operates all the better as the transistor has a high gain; this limits its breakdown voltage and increases breakdown risks;
2) in the case where the component is inserted in a circuit where the biasing between terminals A and B is likely to be reversed, a diode has to be arranged in series with the transistor to avoid destroying the transistor by high reverse currents; this adds to the component cost and increases its power consumption;
3) although current Ie is effectively substantially constant, as mentioned previously, current I coming out from terminal B is equal to the sum of current Ie and of the current flowing through resistor R1 and diode Z which, in operation, is in the on state. Current I depends on VAB, the voltage between terminals A and B, and thus is not constant. The system thus has an overall characteristic IABVAB, where IAB is the current between terminals A and B, substantially such as shown in FIG. 2, that is, the output current varies non-negligibly with the voltage thereacross.
This circuit is however widely used and preferred to depletion MOS-type transistors with interconnected gates and sources, because the limiting current can then be easily set by external components (Vz, R1 . . . ), whereas, in the case of the MOS transistor, the adjustment parameters are internal (channel resistance, threshold voltage . . . ).